Conventionally, it has been a common practice that miniature or small-sized electrical elements are preliminarily attached, at a predetermined pitch or interval, to a lead frame of a thin sheet blanked to form a large number of terminals as shown in FIG. 1. Subsequently, the electrical elements thus prepared are formed into a package by a thermosetting resin material such as phenolic resin, epoxy resin and the like so as to be made into product main body resin portions (2) as shown in a side elevational view of FIG. 2, and thereafter, formed into finished products through processes for cutting off the terminal portions along one-dotted chain lines, trimming at opposite selvadge or edge portions, and finally folding the respective terminals along side faces of said resin portion (2). In connection with the above, the packaging work by the synthetic resin material as described above is normally effected on a large scale through mass production by the employment of a transfer molding press in which metal molds for packaging are set.
FIG. 3 shows such packages formed by a conventional apparatus in a state as they are withdrawn from metal molds, with a portion at the upper right equivalent to slightly over 1/4 of the entire drawing being represented in a top plan view for convenience of illustration. In FIG. 3, there is provided a cull portion (3) formed at a sprue located at the central portion of the metal molds, and connected to respective encapsulation molded resin portions (2) through portions each formed by gates (4) and runners (5) of the metal molds.
Incidentally, in the metal molds, shallow recesses into-which the sprue (3), runners (5), gates (4) and lead frames (1) are fitted, are mainly formed in a lower mold, while cavities into which the resin material is injected under pressure to form the product main body resin portions (2), are formed in an upper mold and the lower mold. The upper mold is further formed with a through-opening in a position immediately above the sprue (3) of the lower mold, to which opening, a lower half portion of a pot for charging the resin material thereinto is fixedly connected. The resin material as described above heated in the pot so as to be in a semi-molten state is injected into the cavities referred to above by an extruding plunger connected to a rod of a pressing cylinder of a molding press through the sprue (3), runners (5) and gates (4) so as to form the respective encapsulation molded resin portions (2) through pressurization. In the above case, since the upper and lower molds of the metal molds are respectively clamped in contact with an upper plate and a lower plate in which heaters are embedded, these upper and lower molds are heated up to a predetermined temperature through heat conduction, but, between one resin portion (2) at a position A which is molded at the cavity closest to the pot i.e. the sprue (3) and another resin portion (2) at a position B to be molded at the cavity farthest from the sprue (3) on the contrary, there is a difference in the time required for the resin material to reach there. Accordingly, owing to the necessity for the resin material which has flowed into the cavity at the position B to retain a fluidity required for the pressure molding, it is so arranged that a filler which delays the hardening time i.e. which increases the fluidity index is mixed into the whole resin material. With the use of the molten resin material as described above, the resin material is poured under pressure from the pot into the respective cavities through the plunger referred to earlier so as to be subjected to a pressure molding, with a subsequent curing to achieve sufficient hardening, and thus, in the conventional practice, at least 2.5 to 3.0 minutes have been required for one packaging work. In the example as shown in FIG. 3, the encapsulation molded resin portions (2) of 14.times.2.times.3.times.4=336 pieces in number are to be formed by the one packaging operation as described above, but as may be assumed from the foregoing description, such conventional arrangement has drawbacks as raised hereinbelow.
(I) Since the conditions should be so arranged that the encapsulation molding at the cavity located in a position farthest from the pot can be perfectly effected, the cycle time for the encapsulation molding tends to be prolonged.
(II) Due to the prolonged cycle time for the encapsulation molding as described above, the number of cavities must be increased for improving the productivity, and as a result, metal molds tend to be large in size, with a consequent rise in the manufacturing cost, while, owing to the necessity to employ a large-sized molding press for setting the bulky metal molds, installation cost may also be increased undesirably.
(III) Since the length of the runners is increased, loss of the synthetic resin material is also increased, and thus, the ratio of the total product weight to the total material weight is low at 55 to 60%.
Accordingly, an essential object of the present invention is to provide an improved encapsulation molding apparatus in which the disadvantages in the conventional encapsulation molding apparatuses have been substantially eliminated.